Free Duct Sizing Calculator
Calculate the required duct diameter for a given airflow rate and maximum velocity.
L/s
m/s
Required Circular Duct Diameter
252 mm
Minimum Duct Area50,000 mm^2
Actual Velocity at Selected Size6.0 m/s
Required Circular Duct Diameter vs Airflow Rate (Q)
HVAC Duct Sizing
Duct size is determined by the airflow requirement and an acceptable air velocity.
Formula
A = Q / V (required area)
D = sqrt(4Q / (pi V)) (circular duct diameter)
where Q is airflow in m^3/s and V is maximum velocity in m/s. Higher velocity means smaller ducts but more noise and pressure drop.
Example Calculation
300 L/s airflow with 6 m/s maximum velocity.
- 01Q = 300 L/s = 0.30 m^3/s
- 02A = 0.30 / 6 = 0.05 m^2 = 50,000 mm^2
- 03D = sqrt(4 x 0.30 / (pi x 6)) x 1000 = sqrt(0.0637) x 1000 = 252 mm
- 04Round up to next standard size: 250 or 300 mm duct.
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